400/set?! how many cams you got, 5?! Cam gears are dirt cheap...and they are used to adjust the lift of the cams by rotating them + or - degrees. Best used when on a dyno and they can readilt adjust them for optimal performance.

They're just used to advance/retard the camshaft timing in relation to the crankshaft. If you're mostly stock, they'll help some- but if you're running aftermarket cams, they can be very helpful for extracting maximum power. You should be able to get some pretty good ones for about $200 a set. $400 is too high.

They'll definitely help on a turbo application. You generally want to decrease overlap between the intake and exhaust events on a turbocharged engine so you don't bleed as much boost away as you would with the stock setting, so yes- they can help quite a bit under boost, even with stock cams.

$400 isn't too steep for some makes of cam gears. Some of them are easily that much. I only paid $185 for my Skunk2s brand new though... look around for some sales or group buys, and you should be able to get a set of the same for much less.